1. Field of the Invention
The present invention relates to automatic methods for focus and astigmatism corrections in a scanning charged-particle beam instrument, such as a scanning electron microscope, having functions of automatic focus correction and automatic astigmatic correction.
2. Description of Related Art
A scanning electron microscope, which is a typical example of scanning charged-particle beam instrument, is now described. In a scanning electron microscope, an electron beam produced from an electron gun is accelerated toward a specimen. The accelerated beam is sharply focused onto the specimen by a condenser lens and an objective lens. The beam is also scanned across a desired area on the specimen by deflection coils. The strength of the beam, i.e., the amount of beam current, is adjusted by the condenser lens and an objective lens aperture (baffle). In this microscope, secondary electrons produced by scanning the specimen with the electron beam are detected. The resulting signal is fed to a CRT in synchronism with the scanning of the primary electron beam. As a result, a scanned image of the specimen is displayed.
The scanning electron microscope constructed in this way has functions of automatic focus correction and automatic astigmatic correction to permit high-resolution imaging. The scanning electron microscope is now further described taking account of these functions. The electron beam from the electron gun is scanned back and forth by the upper and lower stages of deflection coils. In particular, the beam deflected by the upper stage of deflection coil is deflected in the reverse direction by the lower stage of deflection coil. The center point of the deflection of the beam is set close to the principal plane of the objective lens. Sawtooth-wave signals are supplied to the deflection coils from vertical and horizontal scanning signal-generating circuits, so that the beam scans over the specimen in two dimensions. In some cases, triangular-wave signals may be used as the signals for scanning.
As the electron beam is scanned over the specimen, secondary electrons, back-scattered electrons, X-rays, and so on are produced from the specimen. These particles and rays are detected by a detector having characteristics responsive to the kinds of the particles and rays. For example, secondary electrons from the specimen are detected by a secondary electron detector.
The output signal from the detector is stored as an XY two-dimensional image signal in or on an image storage means, such as an image memory or film, in synchronism with the vertical and horizontal scanning signals. The image signal is sent to an image processor, where the signal is processed to permit the observer's easy observation. Then, the data is stored in a control computer, which displays a scanned image of the two-dimensional scanned area of the specimen on the screen of the monitor according to the stored image data.
Also, where the height of the specimen surface varies according to the observation position, it is necessary to control the excitation current either of the objective lens or of an auxiliary focusing coil mounted close to the objective lens, in order to focus the beam onto the specimen surface accurately.
Furthermore, a stigmator coil for correcting astigmatism attributed to nonuniformity of the lens field is usually mounted above the objective lens. A sharp scanned image can be obtained by controlling the stigmator coil. The aforementioned lenses and coils are controlled by controlling the lens control power supply and coil control power supply under control of the control computer.
The instrument is so designed that the objective lens (or the auxiliary focusing coil) and the stigmator coil are controlled by the operator's operations on the control computer. The operator manipulates an encoder knob or the like such that the image becomes sharpest. To simplify the operations, a scanning charged-particle beam instrument, such as a scanning electron microscope, is generally equipped with an automatic control function using an electrical circuit or computer. This function is known as automatic focus and astigmatism correcting functions and described, for example, in U.S. Pat. No. 5,313,062.
The automatic focus correcting function is described while taking a scanning electron microscope as an example. Similar operations are performed also in other scanning ion beam instruments.
First, the amount of excitation B(t) of the objective lens or auxiliary focusing coil is varied in steps. At each value of the excitation, the electron beam is scanned over the specimen. Secondary electrons produced from the specimen by each scan are detected. The resulting signal is supplied to the image processor and passed through a high-pass filter to extract an evaluation value S(t) indicating the sharpness of the image, i.e., the degree of focusing. S(t) is herein referred to as the evaluation value of the focus.
The relation between the amount of excitation B(t) and the evaluation value S(t) is examined using an appropriate function, such as a Gaussian distribution function, until a fit is found. A position t=J where the evaluation value S(t) becomes equal to an extreme value S(J) is found. The J is not restricted to integers but found as a real number. FIG. 1(a) is a graph showing the relation between the amount of excitation B(t) and the evaluation value S(t). In this graph, the horizontal axis indicates the amount of excitation B(t) of the focusing coil or objective lens and the vertical axis indicates the evaluation value S(t) of the focus. FIG. 1(b) is a ray diagram showing the manner in which the beam diameter varies with varying the amount of excitation for focusing. Where automatic astigmatic correction is performed, the horizontal axis indicates the amount of excitation of the stigmator coil, while the vertical axis indicates the evaluation value of astigmatism.
The amount of excitation B(J) providing the relation t=J is fed back to the objective lens or auxiliary focusing coil, and then an image signal is gained. At this time, to speed up the operation, signals fed to the deflection coils may be thinned out. Alternatively, a geometrical figure may be drawn to treat the signal obtained by the detection as a one-dimensional signal.
Where the lens has astigmatism, if the automatic focus correcting function is energized, the relation between the amount of excitation B(t) of the lens and the focus evaluation value S(t) is given by FIG. 2(a). That is, two peaks, or upper and lower foci, appear. FIG. 2(b) is a ray diagram showing the amount of excitation for focusing in a case where the lens is free of astigmatism. FIG. 2(c) is a ray diagram showing the amount of excitation for focusing in a case (FIG. 2(a)) where the lens has astigmatism. As shown in FIG. 2(c), the cross sections of the charged-particle beam at the peak positions of the upper and lower foci are focused only in one direction; the cross sections are not focused at all in the other directions. That is, the charged-particle beam delineates a linear elliptical form.
For the sake of convenience, FIG. 2(c) shows a ray diagram in which the upper focus indicates a focused condition achieved in the X-direction and the lower focus indicates a focused condition achieved in the Y-direction. Of course, depending on the magnitude and direction of astigmatism, the upper and lower foci may be Y- and X-direction foci, respectively. The direction in which focusing is achieved may deviate from the X- and Y-directions by some angles.
Where the lens has astigmatism as described above, if the specimen surface is scanned with a charged-particle beam in the X- and Y-directions, and if an image signal is gained, the displayed image is seen to be sharper in a certain direction. This is a so-called line focus image.
In this case, the focal position should be set midway between the positions of the upper and lower foci. At this position, the cross section of the beam becomes a genuine circle, i.e., circle of least confusion. Therefore, this position is known as the position of the circle of least confusion. The cross section of the charged-particle beam in the specimen position is shaped into a minimum genuine circle by setting the amount of excitation of the objective lens or the amount of excitation of the focusing coil to an amount corresponding to the position of the circle of least confusion and then optimally adjusting the excitation of the stigmator coil. The image obtained by scanning the beam in this way is sharpest.
The automatic astigmatic correction function is next described. When this function is implemented, what is controlled is only the stigmator (stigmator coil). The operation is similar to the automatic focus correction function. This astigmatic correction function is described by referring to FIGS. 3(a)–3(d).
Referring to FIG. 3(a), showing the state assumed before astigmatism is corrected, it can be regarded that adjustment of the excitation of the stigmator coil varies the distance between the position Zm of the circle of least confusion and the position Zx of the X focal point or the distance between the position Zm of the circle of least confusion and the Y focal position Zy.
Generally, a quadrupole coil is used for astigmatic correction. If the excitation of the stigmator coil is adjusted, the focal position, Zx and Zy, moves in the Z-direction while the distance Dx (=Zm−Zx) between the X focal position Zx and the position Zm of the circle of least confusion and the distance Dy (=Zm−Zy) between the Y focal position Zy and the position Zm of the circle of least confusion maintain the relation Dx=Dy. FIG. 3(b) shows the manner in which the X and Y focal positions are moved away from the position of the circle of least confusion. FIG. 3(c) shows the manner in which the X and Y focal positions are moved toward the position of the circle of least confusion. In FIG. 3(b), the radius of the circle of least confusion increases. On the other hand, in FIG. 3(c), the radius decreases. As a result, it can be said that the corrective operation of the quadrupole correcting coil varies the radius of the circle of least confusion without moving its position.
When the amount of correction is increased from the state of FIG. 3(c) and the electron optics assumes the state shown in FIG. 3(d), the relation Dx=Dy=0 holds. That is, all of the X focal position, Y focal position, and position of the circle of least confusion agree. As a result, the radius of the circle of least confusion is minimized. The amount of excitation necessary for the stigmator coil at this time is an optimum amount of excitation.
In the series of operations described so far, the relation between the amount of excitation of the stigmator coil and the evaluation value of astigmatism is coincident with the S(t)-B(t) curve of FIG. 1(a). Therefore, the position at which the relation t=J holds corresponds to the optimum amount of excitation of the stigmator coil.
When the focal position is not coincident with the position of the circle of least confusion, if the excitation of the stigmator coil is adjusted, a focus in the X-direction is obtained when the relation Dz=Dx holds, where Dz is the distance between the present focal position and the position of the circle of least confusion. A focus in the Y-direction is obtained when the relation Dz=Dy holds. That is, a linear focused image appears twice when the excitation of the stigmator coil is adjusted. At this time, the relation between the amount of excitation of the stigmator coil and the evaluation value of astigmatism agrees with the S(t) versus B(t) curve of FIG. 2(a). That is, the midpoint between the amounts of excitation at which the two linear focused images appear respectively is the optimum amount of excitation of the stigmator coil.
As described so far, the sharpest image is obtained in principle by performing one automatic operation for correcting the focus and one automatic operation for correcting the astigmatism.
The conventional automatic method for focus correction described so far has at least seven drawbacks as described below.
First, where there is astigmatism, it has been difficult to determine the focus. That is, where there is astigmatism, if the focus correcting coil is operated, the two peaks at the upper and lower foci are summed up. Two peaks appear (double peak) on the focus evaluation value curve as shown in FIG. 2(a). In this case, the peaks are overlapped in a manner different depending on the amount of astigmatism. Therefore, as the upper and lower foci approach each other, it becomes more difficult to separate the two peaks apart.
The presence or absence of astigmatism is judged according to whether the curve indicating the evaluation value of the focus has a single or double peak. In the case of a double peak, the optimum amount of excitation is set at the center of the double peak. In the case of a single peak, the amount is set at the vertex of the peak. It is difficult, however, to make such a judgment. The vertex on one side of the double peak may be misregarded as the vertex of a single peak, leading to failure to find the optimum amount of excitation.
Second, it is difficult to make astigmatic correction under the state of focus deviation. Under this condition, if the stigmator coil is operated, a double peak consisting of superimposition of the two peaks at the upper and lower foci appears on the curve indicating the evaluation value of the focus. In this case, as the amount of the focus deviation increases, the double peaked curve widens more. As a result, it may be impossible to separate the two peaks. If the curve widens excessively, the detection itself of the peaks is made impossible. In consequence, astigmatic correction is quite difficult to achieve under the state of focus deviation.
Third, it is difficult to judge whether there is astigmatism or not. That is, as mentioned previously, it is difficult to judge whether there is astigmatism because it is difficult to separate the two peaks, especially when the amount of astigmatism is small.
Fourth, when the specimen has a pattern consisting of elements spaced apart from each other in one direction (e.g., a line and space pattern), it has been impossible to recognize that astigmatic correction cannot be done. In particular, where the elements of the specimen are arrayed in one direction, it is theoretically impossible to perform astigmatic correction. However, a clear peak appears on the curve indicating the evaluation amount of astigmatism. Although this peak might be on one side of a double peak, the single peak has been judged as giving an optimum value.
Fifth, the specimen for which the automatic function of astigmatic correction is used has needed to have uniform directionality and many features. That is, where there is astigmatism, the two peaks at the upper and lower foci are superimposed to thereby produce a double peak. The heights of the two peaks may differ depending on the feature of the specimen. This may lead to failure of the detection of the peaks.
For example, where the surface topography of the specimen has strong directionality in the X-direction, a steep peak appears at one of the upper and lower foci. A mild peak appears at the other. If two peaks are superimposed, only the steep peak is conspicuous. Therefore, the obtained amount of excitation is not an optimum value but an amount of excitation at the upper or lower focus. Hence, astigmatic correction has been done unsuccessfully.
Sixth, where the specimen is flat and has less features but has strong directionality like an LSI pattern, it is difficult to enhance the accuracy of automatic focus correction or astigmatic correction. For this reason, there is the danger that an incorrect optimum excitation position is detected. In the case of such a specimen, the image yields a small amount of features. The evaluation value of the focus has been buried in the image noise.
Seventh, where the specimen is flat and has less features but has strong directionality like an LSI pattern, the direction along which the evaluation value of focus is found does not agree with the direction of the pattern. Therefore, the curve obtained by plotting the focus evaluation value against the objective lens evaluation value becomes milder. The optimum amount of focus correction cannot be found accurately. Furthermore, the direction along which the astigmatism evaluation value is found does not agree with the direction of the pattern. The curve obtained by plotting the astigmatism evaluation amount against the stigmator coil excitation amount becomes milder. Consequently, the optimum amount of astigmatic correction cannot be found accurately.